An Afghan official says Pakistani forces have crossed into eastern Afghanistan and clashed with Afghan troops.
Col Abdul Hanan, the acting provincial police chief in the eastern Khost province, says the fighting broke out today and is still underway. He was not immediately able to confirm reports of casualties.
The two countries are separated by the 2,400-kilometer (1,500-mile) Durand Line, which was drawn by British rulers in 1896.
Kabul does not recognize it as an international border and has objected to new fortifications being built by Pakistan.
The two US allies routinely accuse each other of failing to crack down on militants who operate along the porous border.